High frequency transmitting circuit



* 2 Sheets-Sheet l Aug. 18, 1936. G. L. ussELMAN HIGH FREQUENCYTRANSMITTING" CIRCUIT' Filed June 22, 1932 Ildfla. lian- BY /g ATTORNEYAuf# 18| l936- G. L. ussELMAN l V 2,051,503

HIGH FREQUENCY TRANSMITTING CIRCUIT milf TRA//sM/rfm 172g. a Y

\f INVEN'TOR TMW/7750 GEORGE L. ussELMAN y n f BY ATTORNEY Patented Aug.18, 1936 HIGH FREQUENCY TRANSMITTING CIRC UIT

George L. Usselman, Port Jefferson, N. Y., assignor to Radio Corporationof America, a corporation of Delaware Application June 22, 1932, SerialNo. 618,607

11 Claims.

This invention concerns an improved coupling arrangement for couplingthe output of a radio transmitter to a utilization circuit, such as atransmission line or an antenna system.

Radio transmitters are frequently provided with two or more antennasystems for enabling the transmission of signals eiciently on more thanone wave length. It is customary in transmitters which are adapted tooperate on two or more frequencies to use a transmission line andantenna for each frequency and to provide individual variablearrangements for adjusting the coupling of each line. Heretofore,inductive coupling has been avoided in such systems due to the greatdiiiiculty which has been experienced in combining more than oneinductive coupling coil to the single output of the transmitter. Sinceeach of the transmission lines is adapted to function most eiiicientlyon a frequency which differs greatly from those utilized by the othertransmission lines, it has been found that a common coupling coil whichis correct for only one particular frequency and not correct for otherfrequencies, does not lend itself readily to the emcient design ofmulti-frequency radio transmitters. For these reasons, it has been thepractice in the past to use conductive coupling. Conductive coupling,however, has certain disadvantages, such as causing the transmissionline and antenna to radiate second harmonic and push-push parasiticfrequencies. These parasitic frequencies occur because the attachedtransmission line supplies a circuit in addition to the regularoscillating circuit.

In accordance with the present invention, the foregoing disadvantagesare entirely overcome by the provision of a single inductance coilcoupling arrangement for all frequencies and all antennae.

Several features of the present invention reside in the variablereactance circuits for adjusting the couplings of the individualtransmission lines and for efficiently eliminating second harmonicenergy in the lines and antenna systems.

The invention is described more in detail in the following descriptionaccompanied by drawings, in which Figure 1 illustrates a singleinductance coil coupling arrangement in accordance with the presentinvention which is adapted to selectively connect one of a plurality oftransmission lines to the output circuit of a radio transmitting system;and

Figure 2 illustrates a modification of the circuit of Figure 1.

Figures 3, 4 and 5 illustrate preferred embodiments of the presentinvention. 'Ihese include impedance changing circuits for theelimination of undesirable second harmonic energy from the transmissionlines and antenna systems.

Figure 6 illustrates a further embodiment for 54.:

eliminating undesirable second harmonic energy.

Figures 7 and 8 'illustrate the present invention as applied to a singlefrequency transmitter or power amplifier stage utilizing only oneantenna.

Referring to Figure 1, there is shown a multifrequency radiotransmittingV system which is adapted to transmit signals on either oftWo frequencies over antenna'systems I and 2. The

radio transmitter is indicated diagrammatically l5 in box-I and maycomprise a conventional type of push-pull power amplifier stage withwhich there is associated an output coil or tank circuit 2. Since thetransmitting circuit forms no part of the present invention and is of atype that is 20;,A

well known in the art, no detailed descriptionof the transmitter willbegiven herein. A single coil 3 is adapted to inductively couple tankcircuit 2 to any one of the antenna systems l and 2 through switch 4.kConnected to the center point 25; of coil 3 is a high resistanceelement 5 which also connects with the plate supply lead as shown in thedrawings. The purpose'of this resistancewill be described later.Serially connected in the transmission lines Tand 8 'andfextending from30:

switching mechanism 4 tothe antenna systems are a plurality of blockingcondensers 6, 6 of negligible impedance to the high frequency currents,which prevent direct current from the positive source of plate supplyfrom entering the 35 antenna circuits proper.

High frequency radio energy is fed from'tank coil 2 by means of theelectromagnetic coupling to coil 3, through switch 4 to one-of thetransmission line'sl'l andl 8. This switch may be thrown either up ordown to select the desired transmission line and antenna system.

For effecting proper coupling for all transmission lines and antennae attheir operating frequenciescoupling coil 3 is 'adjusted tov give correctcoupling for that transmission line and antenna which takes the mostpower on a particular frequency and, consequently, requires the closestcoupling. In most cases, this amount ofcoupling would be too muchforgcorrect operation of the rsystem on other frequencies over othertransmission lines and antennae. To obviate this diiiculty, adjustableimpedance coils tand I0 are provided in transmission line 8 to` reducethe coupling and to cause the line totake less power.

These impedance coils are then'adjusted to effect Y the desired degreeof coupling. In thisV manner,

the advantages of inductive coupling are obtained by the use of only'asingle coupling coil for a plurality of transmission lines andantennasystems.

Resistor element 5 is arranged to maintain coupling coil 3 Vat thedirect currentV plate potential a to prevent arcing from vthe plate tankcoil V2 to the` coupling coil 3;Y Yarcondition which 'would 10 Yplingzzto any desired'degree.

' InFigui-.e QSiIthere is shown 'an arrangement Ywhich hastbeenusedtofgood 'advantage in'elimi-V otherwise occur due to leakage acrossthe transmission line blocking Vcondens'ers 6, 6.V If it were Anot forthis connection toV the platesuppiy such arcing would be recurrentY asthe charges on the, 1 Y 3 Y coil leak orf through the condensers, 6 andtend to cause the coil to assumejaggroun'd potential.

Figure 2 shows an arrangement wherein a con- 'denser Il, preferablyvariable, is utilized across transmission Aline Il and coupiingicoi1fi`3 for vadjusting the coupling. In this circuit, ,coupling Ycoil3 is arranged to work into a lower impedance and :condenser I I:is-utilized toiincrease the counating undesirable fsecond harmonicenergy in Y' the :transmission :line and antenna. Tori-accomcondenser.21| iisccnnectedftorgroimd:at its center pointiathrongh lead -'i'2.@It-should be notedtthat vso this connection does not affect thefundamental energy inthe:.-lineA which'operates push-pull, that is,inzphase opposition,whereas:all :even harmonic energy; noticeably thesecond harmonic, will be grounded ,rbecause fci'the"push-.push vor'cophasal Y inductance ifmaylbe utilized in ythe-connection i2' to tuneout the` capacitive impedance-of :con-V denser. 2.I.;for the fsecondharmonic frequencyV so 45.v 1 V,pointof coupling-'coil Vwthrough .la:large capacity n efiectl-onithetransmis'sion line. If desired, an

ast'o make the eiectivesimpedance 'to ground.' for 40: the 'secondharmonic-energy very low. Y, In thisV way;:practically;faillithe secondharmonic -energy present in the transmission line isaby-passed Vtoground. Y .f

` Another lmethod `roi' .reducing 'the Yundesirable secondharmonicfenergyzis toground the center blocking condenserI4,`fas"shown"in'1igure 3,of the drawings.V This condenser should yhaveasuncientlyihigh value towwithstand the plate-voltage 0f .the stage toywhich coil"-3.is coupled. n Y .llfnfligures4;iand.5 are-:shownthe-,preferred--arf Yrangements embodying "the principles -of thepresent invention. 'I'hese embodiments utilize an improved couplingarrangementL comprising 4both `the coilsv9 and @wand the balancedv orsplit capacity type condenserY 2|` `to eiect'fthe desired changev intransmission line :impedance facing Vcoupling `coili. `Goils '9 andvI-ll, Vtogether with :con-

denser 2 L' may'bejcalledian `impedanceichanging circuit,:ani-important. characteristic of which s'its ability when jproperlyadjusted to present :to the coupling coil 3 the :desired impedanceat'unity powenfactor; 11n other swords, the loadpresented tothetransmitterfat the `operating Yfrequency .is purely resistive. Theadjustment .of the ,couplingarrangementshown in :Figures-4 and 5.is

somewhat'diierent from :thatzdescri-bedifor Figures -1,. 2 -and 3.The/.coil 3 '.intFigures 4 vand A5 may lbeconstructed :to :providekcorrect coupling for-either-.onewof thegtransmission vlines and an-'tennae` r impedance changing circuits (coils 9 fand Iilandscondenserlflmay befa'dusted'toV provide correct cnupling'for' the remainingtransmission :lines land antennae. Again, coupling coil.'iumay-:bemadeztorhave any Yconvenient-'or desi-rable size and thecorrect degree of coupling mayV be'provided for each transmission lineand antenna by proper adjustment of an impedance changing circuit (coils9 and I0 and condense 2i) in each transmission line. Y

Switching elements l5 are used for'grounding the idle antennatransmission line. It has been found that the Vharmonic radiation fromVthe transmitter is thus greatly reduced by grounding `in the poweramplifier thatY transmission line' vwhich is notin use at theVparticular time of 'Y transmission. Switches I5, I5, it should benoted, short circuiting switches and made to ground the transmissionlines on the antenna vside of the `D. C. blocking condensers 6, -6..VThese ground switches may `be connected to the frame of the transmitteror to any'point WhichrisV substantially ...at :ground potential.

In Figure 5 variable impedance coils 9 and'lU are' shown vconnectedbetween balanced con-Y denser`c2il,:wh'rch connectedracross thetransmission line, and the antenna, whereas in the other figures theivariable impedance coils are shown between 4-couplingfcoil 3rgai-id`\the'ba1ancedY condenser '-Zl.. :Theformer'manner of connectionis fdesirable'ioridecreasing the-'coupling between theantennaasystemsand the coupling coil,Y

wl'ieneastheatter imannerisdesirable for 'increasing ztiierfcoupling. i

It should .be fnotedthatigures 4 and v5 also innlu'de thelconnection-015 `fthe middle element of variable condenser1-l to'sgroundVthrough the Y varablelinduwctanceccoil :i3 and -the v connection ofthemidpoint'of couplingffcoili3 to groundior highirequencies through thekbit-passv condenser lf4,.forthe :purpose of reducing harmonicradiation; -Y Y Figure 6 :illustratesLaiurther arrangement rforeliminating secondharmonic lenergy in the transmission `Elineandantenna. 'Ihe circuit shown consists loiltwo connectionsy eachhavinga length of fone-fourth of -thegfun'damental wave lengthextendingffrom-fground 'to points Vlli `and H on each 'of the twowires 1respectively, which are the `:samedistance from'coil 8.These'connections are one-quarter wave-'length long for -thefundamentall frequency and thereforel oier `a high impedance-toycurrents ofV this frequency, but are onehazifwaive flengthV long forthe second harmon-ic irequency'andfpresentvery ilo-w impedanceto'rground `for currents lofrthis latter frequency,v thus `Ieliminatingenergy .of this v-harmonic frei Y quency. iromithe' transmission :linesand antennae.

If desired, coils which are effectively a quarter wave length Vlong'forthe'fun'damental frequency cria 'half Wave length v-long'ffor the secondhar-.l

monicifrequency maybe used insteadfof the one quarter lWave "lengthstraight WiresV for eliminatingthe second harmonic energy. l'l'.fstanding Waves exist cophasially on the transmission line,V points 'L6and YIl Ashould be Alocated .at the high potential .points on thereectedsecond harmonic wave for more effective elimination of the second Y inaccordance with the present invention, adjust the coupling to therequired degree. Y

Figures 7 and 8 illustrate a modiiied form o the present invention asapplied to a single frequency transmitter having one antenna system, oras applied to a multi-frequency transmitter having a power amplifierstage for each frequency and antenna. In the iigures shown, both powerampliiiers of a transmitter, of course, may be energized at differenttimes from a single exciter circuit. Figure 'l shows the arrangement ofthe impedance changing circuit 9, 10, 21, for presenting a low impedanceto the coupling coil 3. Such an arrangement is adapted to increase thecoupling between the transmitter and the antenna system. Figure 8 showsthe arrangement of the impedance changing circuit for presenting a highimpedance to coupling coil 3. The latter coupling arrangement decreasesthe coupling between the transmitter and the antenna.

'17ha coupling arrangements shown in Figures 7 and 8 are desirable forreducing harmonic radiation and for stopping parasitic oscillations andalso for enabling the use in the transmitter of a rigidly fixed couplingcoil without fractional turns. This also allows the use of a convenientor desirable size of coupling coil regardless of coupling since correctcoupling can be obtained by proper adjustment of the impedance changingcircuit (coils 9 and l0 and condenser 2l) I claim:

l. A wireless transmitter adapted to operate on either of two or morefrequencies having, in combination, an ampliiier stage, a transformerhaving a primary winding and a secondary winding, said primary windingcomprising the output of said amplifier stage, a first two wiretransmission line and a second two wire transmission line, condensers ineach wire of said two transmission lines, switching mechanism forselectively connecting either one of said transmission lines to saidsecondary Winding of said transformer, and a balanced variable condenserarrangement connected across the wires of one of said transmission linesfor adjusting the impedance thereof, the eiiective midpoint of saidconde ser arrangement being grounded.

2. A wireless transmitter adapted rto operate on either of two or morefrequencies having, in combination, an ampliiier stage, a first two wiretransmission line and a second two wire trans-v mission line, condensersin each wire of said two transmission lines, a single coil inductivelycoupled to the output of said ampliiier stage, switching mechanism forselectively connecting either one of said transmission lines to saidsingle coil, and variable reactance means associated with one of saidtransmission lines for adjusting the coupling thereof, a connection fromthe mid-point of said inductively coupled single coil to ground, andanother connection from the eiTective center point of said variablereactance means to ground.

3. A wireless transmitter having, in combination, an amplifier stage, aiirst two wire transmission line, and a second two wire transmissionline, condensers in each wire of said two transmission lines, a singlecoil inductively coupled to the output of said amplier stage, switchingmechanism for selectively connecting either one of said transmissionlines to said single coil, a variable condenser connected across thewires of one of said transmission lines for adjusting the couplingthereof, a connection from the mid-point of said inductively coupledsingle coil to ground, and another connection from the effective centerpoint of said variable condenser to ground, said last connectionincluding a variable inductance to tune out the capacitive impedance ofsaid variable condenser for the second harmonic frequency of thefundamental working wave for which said one transmission line is tuned.

4. In combination, a wireless transmitter including a power amplifierstage having an output circuit, a single coil inductively coupled tosaid output circuit, a connection including a capacity from themid-point of said single coil to ground, a first antenna circuit and aseco-nd antenna circuit, means for selectively associating said singlecoil in operative relation with either of said two antennas, condensersserially connected in the lines of said first and second antennacircuits, and variable reactance means in one of said antenna circuitsfor adjusting the coupling of said one circuit to said single coil.

5. In combination, a wireless transmitter including a power amplifierstage having an output circuit comprising a coil whose mid-point iscapacitively connected to ground, another coil inductively coupled tosaid rst coil and a capacitive connection from the mid-point of saidsecond coil to ground, a first transmission line and a secondtransmission line, switching mechanism for selectively connecting saidsecond coil with either one of said two transmission lines, and variablereactance means in one of said transmission lines for adjusting thecoupling of said one transmission line to said second coil.

6. A wireless transmission system as defined in claim 5 including aresistor connected between the mid-points of said two coils.

7. A radio frequency transmitter having, in combination, an outputcircuit including a iirst inductance coil, a plurality of transmissionline systems adapted to operate on different frequencies, a second coilinductively coupled to said inductance in the output circuit, switchingmechanism for selectively connecting said second coil to any one of saidtransmission line systems, and means for maintaining the mid points ofboth coils at substantially the same direct current potential.

8. A radio frequency transmitter having, in combination, an outputcircuit including an inductance, a two wire transmission line, anantenna, a single coil inductively coupled to said inductance in saidoutput circuit and connected to one end of said line, the other end ofwhich is connected to said antenna, adjustable coupling means comprisinga variable reactance coil in each wire of said line, a balanced variablecondenser across said line, and a connection from the middle of saidbalanced condenser to ground.

9. A transmitter as defined in claim 8 including a variable reactancecoil in said connection for tuning the capacity of said condenser.

10. In combination, a radio frequency transmitter having an outputcircuit, a coupling coil inductively related to said output circuit, anantenna having a two conductor transmission line connecting said antennawith said coupling coil, an impedance changing circuit having aninductive reactance coil connecting one of said transmission lineconductors to one terminal of said coupling coil, a second inductivereactance coil connecting the second transmission line conductor to theother terminal of said coupling coil, and a capacitive reactancecondenser having a central element connected to ground through aductorsl said reactance Ycoils `and condenser of said impedance'changingcircuit beingw'capable of Y arrangement and'adjustment totransformat-the operating frequency the transmission line-iltmrned- Yance in such manneras topr-esent to the-coupling coil and transmitteroutput circuit a load ofpure resistance and` of optimum valuerlfnatving`minimum harmonicradiation.'V Y

Y 11. In combination, a wireless transmitter including a power amplifierstage having an output circuit, a single .colrinductively coupled tosaid output circuit, azrst antenna circuit and a seceither--ofjsaidtworrantennas, condensers serially Vconnected in the lines ofsaid'rflrst and second an'-,

tenna circuits, and variable reactance means in one of said antennacircuits for adjusting theVV coupling of said one circuit to said singlecoil.

GEORGE YI... UssELMAN.

' ond antenna circuit, means for selectively asso- Y Vciating saidsingle coilin operative relation with Y

